1. Field of the Invention
The present invention relates to a moving picture coding apparatus applied to television or the like for transmitting a high-quality image.
2. Description of the Related Art
In conventional moving picture coding apparatuses for transmitting moving picture information at a variable bit rate by inter-frame or intra-frame coding, a coded image data (signal) is stored in a buffer for cellulation divided into fixed-length packets, and when cells are formed, is outputted to a channel as a networks data.
An example of the conventional moving picture coding apparatuses is shown in FIG. 1.
In FIG. 1, reference numeral 1 designates an input image frame memory for storing an image, and numeral 2 an operator for computing, for each block, the difference between the image read out of the input image frame memory 1 and the reference image for inter-frame coding read out of the reference image frame memory 5 or "0". Numeral 3 designates a quantizer for quantizing the predictive error, which is the result of operation by the operator 2. Numeral 4 designates an operator for adding the result of quantization by the quantizer 3 and the image stored in the frame memory 5 or "0" to store a new image for next predict in the frame memory 5. Numeral 6 designates a refresh control circuit for deciding whether or not a forcible intra-frame coding (refreshing) should be effected in order to suppress the image degradation against the present position of a coding block supplied by a coding position information line W3, and if refreshing is required, effecting the intraframe coding or, otherwise, controlling a switch 11 through a refresh control line W2 in order to effect the inter-frame coding. Numeral 7 designates an inter/intra coding decision circuit for deciding for each block whether the image in the input image frame memory 1 and the reference image in the frame memory 5 are correlated to each other, and if correlated, effecting the inter-frame coding with motion compensation as mentioning later or otherwise effecting intra-frame coding by selecting switch 11. Numeral 8 designates a motion detection circuit for deciding whether each block of an input image stored in the input image frame memory 1 is a static or dynamic area against the block of the corresponding position of a reference image stored in the frame memory 5. If motion is detected, motion compensation is accomplished so as to minimize the predictive error. Numeral 9 designates a variable-length coding circuit for preparing a variable-length coded data in response to an output from the quantizer 3 and the motion information from the motion detection circuit 8. Numeral 10 designates a packet preparation buffer for preparing packets by packetizing into a predetermined length from the variable-length coded data prepared at the variable-length coding circuit 9. Numeral 11 designates a selecting switch for selecting the inter-frame or intra-frame coding on the basis of the decision of the refresh control circuit 6 and the inter/intra coding decision circuit 7.
According to the above-mentioned conventional moving picture coding apparatuses for transmitting an image data in variable bit rate, cell loss occurred when congestion occurred on the packet switched networks side, therefore the decoded image is degraded at a decoder. In such a case, once information is lost by the cell loss, degradation is promoted along time axis. In similar fashion, upon occurrence of a channel error in the transmission path, the difference in the interframe coding data between transmitting and receiving ends makes it impossible to decode an image correctly at the receiving end. Also, a channel error hinders from the correct reproduction of an image during long subsequent periods. In order to solve these problems, the conventional apparatuses are so constructed as to reduce the degradation along time axis by refreshing which is a forcible intra-frame coding at regular intervals of time.
An example of refreshing operation is shown in FIG. 2. The refresh control circuit 6 switches the operation of the switch 11 to refresh a predetermined amount (two in the case of FIG. 2) of blocks sequentially at intervals of 1/30 seconds. Since the block 3 which has been degraded by cell loss is refreshed at a time point t.sub.2, the degradation is suppressed at a time point t.sub.3.
The problem of the above-described conventional moving picture coding apparatus, however, is that since a predetermined amount of blocks is subjected to a continued intra-frame coding by refreshing with a great amount of information generated, when cell loss occurred, then, there is a case inter-frame coding information is lost, thereby conspicuously degrade the image quality.